OPEN-LABEL TRIAL: A clinical trial in which doctors and participants know which drug or vaccine is being administered. See also Clinical Trial.

OPPORTUNISTIC INFECTION: 1. An illness caused by an organism that usually does not cause disease in a person with a normal immune system. People with advanced HIV infection suffer opportunistic infections of the lungs, brain, eyes and other organs. 2. Opportunistic infections common in AIDS patients include Pneumocystis carinii pneumonia, Kaposi's sarcoma, shigellosis, histoplasmosis and other parasitic, viral, and fungal infections, and some types of cancers. See also Histoplasmosis; Kaposi's Sarcoma; Pneumocystis carinii Pneumonia.